Method of drying



Patented Dec. 13, 1949 METHOD OF DRYING James G. Wiegerink, Ridlley Park, Pm, alsignor to American Viscose Corporation, Wilmington, DeL, a corporation of Delaware No Drawing.

Application May 5, 185,

Serial No. 592.269

1 Claim. (Cl. 34-1) This invention relates to a method of drying I packages of fibrous or filamentary material, such as wound packages of fabrics, felts, yarns, tows, slivers and the like, or thick mats of staple fibers. The invention is applicable to the drying of such wound packages or bulky masses of any fibrous material, natural or artificial, such as cotton, silk, rayon, acetate rayon, other cellulose derivatives, nylon, other synthetic resins including polyvinyl chlorides, copolymers of polyvinyl chlorides with acrylonitrile or vinyl acetate, polyvinylidene chlorides, caseins and other proteins, wool, kapok, etc. However, to simplify the description and to illustrate an especially important application of the method, the invention is hereinafter described particularly with reference to regenerated cellulose filaments and staple fibers obtained from viscose or cuprammonium cellulose solutions.

It has heretofore been suggested to apply high frequency induction, either electromagnetically or electrostatically, to heat and dry packages of rayon, such as the annular wound "cakes" of regenerated cellulose filaments as collected in centrifugal spinnin buckets, after suitable liquid treatment thereof. The induction heating method has heretofore been suggested in order to overcome the difliculties which arise when using ordinary drying methods, such as currents of heated air, because of the fact that the outer windings of the cake tend to shrink during drying, whereas the inner windings oppose such shrinkage because the drying medium does not penetrate to the inside of the cake suiiiciently rapidly to cause it to shrink along with the outer windings. The fact that the inside windings of the cake are allowed free shrinkage, whereas the shrinkage during drying of the outer windings of the cake is opposed, gives rise to strains in the yarns which causes non-uniformity of dyeing of the fabric ultimately produced from the rayon dried by ordinary methods. However, it has been found that the induction method overheats the inside portions of the bulky masses of fibrous material because of the tendency of the vapors developed inside the mass to move to the outer portions thereof. This results in a reversal of the characteristics of the final yarns as compared to those obtained by ordinary drying methods.

Such overheating, however, degrades the material composing the fiber, reduces its strength, discolors it and generally affects the fiber properties disadvantageously. The application of a vacuum to assist the evaporation helps this condition somewhat, but is not in itself a complete answer to the diiliculties, since even when a vacuum condition is imposed upon the mass being dried, there is a definite gradient from the middle oi the mass to each surface with respect to the moisture content thereof, so that the outer portions of the mass will still contain moisture and need further drying when the windings within the mass have reached bone-dry condition. Continued application of the induction in order to complete the drying of the outer portions of the mass results in injurious effects upon the inner portions of the mass. 1

In accordance with the present invention, it has been found that drying can be accomplished in an effective economical manner with improved fiber properties in the final dried mass by first heating the mass to be dried by high frequency induction and then completin the drying by ordinary methods, such as by passing heated air or other gaseous medium through the mass. The proportion of moisture removed by the induction applied at the preliminary drying stage may vary widely from a relatively small proportion of about up to but not over 1; of the amount of moisture originally present. Then, the drying is completed to the desired moisture content by the circulation of a hot gaseous medium through the mass. By operating in accordance with the present invention it has been found that the straining and non-uniform dyeing properties of the fibers are substantially entirely eliminated, while the danger of overheating parts of the masses is entirely avoided. The surprising and important fact is that the advantages of induction heating in preventing development of strains because of irregular shrinkage effects is not lost by failure to dry the mass completely by that method. These advantages c'an be preserved and the disadvantages ent invention'also makes it possible to dry larger masses thanwould be possible with high frequency induction alone as the drying agent.

In speaking of drying, or completing the dry ing of, rayon or any other of the fibers, it is intended to cover those situations in which a small proportion of moisture is left within the fibers as well as those in which a condition of so-called "bone dryness or absolute dryness is reached. Rayon ordinarily has a regain of about 11% moisture, and it is the usual practice to dry originally beyond this point, such as to a content of about 5%, though the precise point to which dryin is carried depends upon the particular conditions, such as the size of the package and its" density, the point being chosen to assure that all portions of the mass have been carried to a dryness beyond the normal regain.

Wherever percentage moisture content is stated herein, it is based upon the dry weight of the material, such as regenerated cellulose in the case of rayon, so that a mass containing 5% moisture has 5 parts by weight of moisture per 100 parts of dry mass. Moisture is intended in a generic sense to include any liquid.

The preliminary stage of drying in accordance with the present invention may be accomplished by a high frequency coil electromagnetically coupled to the mass to be dried or by a radio frequency oscillator electrostatically coupled through condenser plates to the mass being dried. During the preliminary stage, a vacuum may be applied, or heated air may be circulated through the mass, or a body of air or other gaseous medium having a controlled humidity may be drawn through the mass. The characteristics of the induction field may be varied in conventional manner during the drying, and likewise the humidity of the medium circulated through the mass during the induction stage of heating may be varied as desired, if such a medium is: used. Likewise, the

gaseous medium circulated through the mass to be dried during the second stage in the absence of high frequency induction may be of any temperature and may have a controlled humidity. However, the advantages of the present invention are generally attainable without the use of controlled humidity and without special care in adlusting the strength of induction field and in varying the temperature of the medium used in the final drying stage. Itis for this additional reason that the efficiency of the present invention from a cost standpoint is high.

As stated hereinabove, the high frequency induction heating during the preliminary stage may be continued until all but Vs of the moisture originally present is removed from the fibers. figure is based upon a starting condition in which the regenerated cellulose is in a highly swollen condition as it comes from the wet treatment stages of an artificial filament spinning procedure and before any substantial amount of drying has been performed upon the material. Generally the lower or ultimate limit to which the high frequency induction can be carried in accordance with the invention is about 30% moisture content based on the dry weight of the fibers. This precise figure may vary somewhat, depending upon conditions, such as the size of the package and the characteristics of the high frequency induction field. However, the preferred upper limit of moisture content range which must be reached or passed by the high frequency stage may be defined as that corresponding to the average moisture content of the mass below which portions of the mass do not have surface moisture as distinguished from water of absorption. By "surface moisture is meant that proportion which is loosely attached to the fiber and is not absorbed in it. When all of the surface moisture has been removed from a portion of the This mass, further application of induction heating causes a heating of the rayon itself in that portion which has lost its surface moisture, and this heating requires a greater transfer of energy per unit of moisture removed than that required merely to remove surface moisture.

The following examples, in which the percent moisture is based on the dry weight of the fibers, illustrate the invention.

Example I A batch of high tenacity regenerated cellulose cakes having a moisture content of 130% were dried to a content of moisture between the electrodes of a radio frequency oscillator operated as in Example I, except that no vacuum was used. Again, 15 minutes were required for the induction period and this was followed by air drying at 155 F. to about 5% moisture content.

Example II I A batch of high tenacity regenerated cellulose cakes were dried to a 30% moisture content by radio frequency as in Example II in a period of 30 minutes. This was followed by air drying at 155 F. to 5% moisture content.

Example IV A batch of regenerated cellulose cakes having 190% moisture content were dried to a moisture content of 50% in a period of 30 minutes while subjected to radio frequency at atmospheric pressure. The cakes were then air dried at 155 F. to 5% moisture.

Example V Cakes of high tenacity regenerated cellulose yarns were continuously conveyed by means of a belt in sequence through a chamber containing electrodes connected to a radio frequency oscillator disposed on opposite sides of the belt and then through a chamber in which air heated to 155 F. was circulated. The speed of the belt was so adjusted with respect to the distinct heating stages that the cakes took a period of approximately 20 minutes in passing between the radio frequency electrodes, during which they were dried to a 60% moisture content from an initial moisture content and during the subsequent stage took a period of about 20 hours through the air drying tunnel, during which their moisture content was reduced to 5%.

Example VI A cake of denier 40 filament delustered regenerated cellulose yarn having a moisture content of was dried to a moisture content of 100% by an electromagnetic induction coil. Then air at 135 F. was circulated through the cake until a moisture content of 5% was attained.

The invention is applicable equally as well to batch and continuous handling methods. This is a special advantage derived from the fact that the present method is not dependent upon the use of a vacuum in order to obtain fibers of improved quality. One advantageous embodiment involves the continuous passage of a belt preferably permeable to vapor carrying the masses to be dried, such as the rayon cakes, at a constant linear speed through a long tunnel, in the first portion of which electrodes may be disposed on opposite sides of the belt or above and below the belt, or coils may be provided to encircle the belt and the last portion of the tunnel is merely prov ded with conventional means for circulatin heat d air through the cakes upon the belt. Alternatively other conveying means. such as chain-driven rods may be used for supportin the masses to be dried. The exhaust steam from' the powerhouse used for generating the electrical current used in the first stage of the heating, may be used for heating the air in the second stage of the tunnel. Since the preliminary drying is done by means of high frequency induction, and since that stage prevents later development of strains due to irregular shrinkage, the final stage of drying by ordinary means, suchas circulation of heated air, may be done more rapidly than is ordinarily the case in conventional tunnel dryers using heated air or other gaseous medium. Thus whereas temperatures of 140 to 155 F. are the conventional ones used. the dual drying stage in accordance with this invention may involve air heated to 100 F. up to as high as 200 F. without encountering trouble with dyeing and other irregular effects upon the fiber characteristics.

It is to be understood that changes and variainitial moisture content has been removed, in-' terrupting the induction heating before the moisture content of the package has been reduced below one-fifth the initial content, and subsequently removing the remaining moisture by circulation of a heated gaseous medium with respect to the package. JAMES G. WIEGERINK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,042,145 Darrah May 26, 1936 2,231,457 Stephen Feb. 11, 1941 2,325,652 Bierwirth Aug. 3, 1943 2,403,800 Hoyler July 9, 1946 2,421,334 Kline et a1. a' May27, 1947 2,428,615 Brown Oct. 7, 1947 OTHER.

Drying and Laminating by Radio Frequency," by George F. Russell, in The Timber-man," Autions may be made without departing from the gust 1944 issue, pages 38, 40, 42. 

